The major reason that adolescents have different sensitivities to alcohol compared to adults is that their brains are still maturing. Although it was once thought that the brain is fully mature around birth this hypothesis has been disproven; now there is clear evidence that the brain does not mature fully until about age 24. One of the areas of the brain that matures late is the prefrontal cortex the area important in impulse control risk-taking behavior and judgment.

During development in the womb as many as 250,000 new neurons (the major cells in the brain) are created each day. These neurons use spatial and chemical cues to find their synaptic targets. By the time we are born our brains contain billions of neurons with trillions of connections. However the infant brain contains far more neurons than are present in the adult brain.

During the subsequent months and through adolescence careful pruning of neuronal connections eliminates all but the most “useful” connections between neurons. The result is a “thinning-out” process that selects for those neuronal connections strengthened through repeated experience. In this sense “cells that fire together wire together” while those that do not make meaningful contacts do not survive. In other words “use it or lose it”! These early pruning processes not only establish the neuronal networks to support learning throughout life but also allow the brain to be “sculpted” based on a person’s unique experiences.

Thus one might imagine how the presence of alcohol can interfere with this time-critical process of neuron pruning that forms the adult brain.

Figure 3.1 The brain undergoes maturation over the first 24 years of life; the sculpting and pruning of neurons and synapses is indicated by the decrease in gray matter (in red) in the cerebral cortex. (Courtesy of Gogtay et al. with permission). For an animated version click here.